1. Field of the Invention
The present invention relates to a recording apparatus for recording images with the beam of a laser printer or the like.
2. Related Background Art
FIG. 7 is a cross-sectional view illustrating a drum-type laser printer. A printer body 41 comprises a paper-feeding section, an image-forming section, a laser exposure section, a conveying section, a fixing section, and a paper discharge section.
A paper-feeding cassette 42 is adapted to feed recording paper 43 into the printer body 41 by the rotation of a feed roller 44. A pair of registration rollers 45 temporarily stop the recording paper 43 which has been fed, and, after adjusting the timing with the tip of an image on a photosensitive drum 51M, feed the recording sheet 43. A conveying belt 46 conveys the fed recording paper 43 in a flat state. Chargers 47, 48 apply a high voltage to the conveyed recording paper 43 and thereby cause the recording sheet 43 to be attracted to and carried on a conveying belt 46.
A laser unit 49M applies a laser beam modulated in such a manner as to be turned ON and OFF in correspondence with a magenta signal from among the image signals transmitted from an external device, so as to scan the photosensitive drum 51M. A developer 50M develops with a magenta toner an electrostatic latent image for magenta formed on the photosensitive drum 51M. A charger 52M charges the photosensitive drum 51M uniformly before image formation. A cleaner section 53M recovers the magenta toner remaining in the photosensitive drum 51M and cleans the photosensitive drum 51M. A transfer charger 54M transfers the magenta image developed on the photosensitive drum 51M onto the recording paper 43 being conveyed. A magenta station MS comprises the above-mentioned components 49M-54M.
A laser unit 49C applies a laser beam modulated in such a manner as to be turned ON and OFF in correspondence with a cyan signal from among the image signals transmitted from an external device, so as to scan the photosensitive drum 51C. A developer 50C develops with a cyan toner an electrostatic latent image for cyan formed on the photosensitive drum 51C. A charger 52C charges the photosensitive drum 51C uniformly before image formation. A cleaner section 53C recovers the cyan toner remaining in the photosensitive drum 51C to clean the photosensitive drum 51C. A transfer charger 54C transfers the cyan image developed on the photosensitive drum 51C onto the recording paper 43 being conveyed A cyan station CS comprises the above-mentioned components 49C-54C.
A laser unit 49Y applies a laser beam modulated in such a manner as to be turned ON and OFF in correspondence with a yellow signal from among the image signals transmitted from an external device, so as to scan the photosensitive drum 51Y. A developer 50Y develops with a yellow toner an electrostatic latent image for yellow formed on the photosensitive drum 51Y. A charger 52Y charges the photosensitive drum 51Y uniformly before image formation. A cleaner section 53Y recovers the yellow toner remaining in the photosensitive drum 51Y to clean the photosensitive drum 51Y. A transfer charger 54Y transfers the yellow image developed on the photosensitive drum 51Y onto the recording paper 43 being conveyed. A yellow station YS comprises the above-mentioned components 49Y-54Y.
A laser unit 49BK applies a laser beam modulated in such a manner as to be turned ON and OFF in correspondence with a black signal from among the image signals transmitted from an external device, so as to scan the photosensitive drum 51BK. A developer 50BK develops with a black toner an electrostatic latent image for black formed on the photosensitive drum 51BK. A charger 52BK charges the photosensitive drum 51BK uniformly before image formation. A cleaner section 53BK recovers the black toner remaining in the photosensitive drum 51BK to clean the photosensitive drum 51BK. A transfer charger 54BK transfers the black image developed on the photosensitive drum 51BK onto the recording paper 43 being conveyed. A black station BKS comprises the above-mentioned components 49BK-54BK.
The recording paper 43 onto which the four color toners have been transferred is thermally compressed by the fixer 55, with the result that a color image is fixed on the recording paper 43. If this fixing process is completed, the recording paper 43 is discharged from the printer body 41, and is placed on a discharge tray 56.
FIG. 8 is a perspective view illustrating a laser beam scanning process using the laser unit shown in FIG. 7, the same components as those shown in FIG. 7 being denoted by the same reference numerals.
In this drawing, a scanner motor 61BK is adapted to rotate a polygon mirror 62BK constituted by a 10-face mirror in the direction of the arrow at a fixed speed. A semiconductor laser 63BK is modulated in such a manner as to be turned ON and OFF in accordance with an image signal input. A cylindrical lens 64BK directs a laser beam emitted from the semiconductor laser 63BK and applies the same to the polygon mirror 62BK. An f/.theta. lens 65BK causes the laser beam deflected by the polygon mirror 62BK to effect horizontal scanning at a uniform speed with respect to the axial direction of the photosensitive drum 51 BK. A reflection mirror 66BK introduces the laser beam to be deflected into a beam detection sensor (BD sensor) 67BK. The BD sensor generates a horizontal synchronization signal which serves as a reference for writing in the main scanning direction (horizontal direction) of the photosensitive drum 51M, and delivers its output to a printer controlling section (not shown). It should be noted that, although a description has been given of an arrangement of the laser unit by using the black station BKS as an example, the other stations are provided with identical arrangements.
FIG. 9 is a top plan view illustrating an arrangement for detecting the rotating speed of the polygon mirror 62BK shown in FIG. 8. In FIG. 9, the same components as those shown in FIG. 8 are denoted by the same reference numerals.
In this drawing, an FG (frequency generator) sensor 68BK outputs a speed detection signal FG to a PLL (phase lock loop) circuit (constituted by a device such as a PLLIC (phase lock loop integrating circuit)) for controlling the rotation of the scanner motor 61BK. A slit encoder 69, which is provided with 10 slits 70 (corresponding to the number of sides of polygon mirror 62BK), as illustrated in the drawing, is secured to a rotating shaft of the scanner motor 61BK and is adapted to rotate at the same speed as the polygon mirror 62BK. As the slit encoder 69 passes the position of the FG sensor 68BK ten times during one rotation, the frequency is measured. The slits 70 are constituted by, for example, magnets, which can be detected by the FG sensor 68BK which is constituted by an electromagnetic pickup or the like.
If the frequency of the rotating shaft of the scanner motor 61BK is, for instance, 12,000 rpm, an FG frequency (f.sub.FG) is determined by the following formula (1): EQU f.sub.FG =12,000.times.1/60.times.10=2 (kHz) (1)
Accordingly, if the reference frequency to be input to the PLL circuit is set to 2 (kHz) obtained in Formula (1) above, it becomes possible to effect the synchronization of rotation whereby both the number of revolutions and a phase corresponding to the reference frequency are synchronized.
Therefore, if a common reference oscillator is used for controlling the rotation of the polygon mirrors in the stations and if the motors are driven by a common reference oscillator, the polygon mirrors of the all the stations rotate synchronously. Hence, it becomes possible to improve the accuracy in registration.
However, in order to meet the aforementioned relationship, the installation positions of the BD sensor 67BK and the FG sensor 68BK must be set in such a manner as to be aligned with the surface position of the polygon mirror 62BK. Moreover, the same installation accuracy is required of the other stations as well. 10 However, in the light of the assembly process, it is impossible to effect installation in a state which meets the aforementioned relationship.
For this reason, the top margin of an image formed by the photosensitive drums 51M, 51C, 51Y and 51BK with respect to the recording paper 43 varies slightly. Furthermore, concerning the accuracy of the installation interval of the stations, it is difficult to install them in units of 63.5 .mu.m (equivalent to the width of one picture element), and the factor of variation in the top margin adds to this difficulty.
Accordingly, adjustment of such a top margin can be effected by adjusting the writing timing of the laser beam.
With such an adjusting method, however, the adjusting units depend on the interval of horizontal scanning, i.e., the laser beam performs horizontal scanning at a predetermined interval (the interval of one picture element). As a result, the unit of adjustment, for instance, between the starting position of writing a black image and the starting position of writing a magenta image inevitably becomes the unit of one picture element, so that color offsetting of a maximum of a 1/2 picture element occurs.
Consequently, particularly when a fine black character image is formed, there has been a problem in that the black-character images cannot be formed with excellent reproducibility.
If an attempt is made to overcome such a problem through mechanical precision alone, since the unit of one picture element requires fine adjustments, it is impossible to follow its variation by periodical inspection or the like alone. Thus, the situation has been such that there has been a strong demand for overcoming this problem.
Hence, as measures for overcoming the offsetting of the position of an image, the present applicant filed U.S. application Ser. No. 149526 (filed Jan. 28, 1988), U.S. application Ser. No. 187,078 (filed Apr. 27, 1988), and U.S. application Ser. No. 195,802 (filed May 19, 1988). However, there has been demand for further improvements.